Project Euler - Problem 26 Redux (using Parallel)

The original post Project Euler - Problem 26 was done some time ago, and I recently read about using parallel methods to improve performance, and in this case, Parallel reduces execution time by 25%.


Description



A unit fraction contains 1 in the numerator. The decimal representation of the unit fractions with denominators 2 to 10 are given:


1/2 = 0.5
1/3 = 0.(3)
1/4 = 0.25
1/5 = 0.2
1/6 = 0.1(6)
1/7 = 0.(142857)
1/8 = 0.125
1/9 = 0.(1)
1/10 = 0.1


Where 0.1(6) means 0.166666..., and has a 1-digit recurring cycle. It can be seen that 1/7 has a 6-digit recurring cycle.


Find the value of d < 1000 for which 1/d contains the longest recurring cycle in its decimal fraction part.


Solution (using Parallel) 


open System
open System.Threading
open System.Threading.Tasks


let rec remainders (num:int) (rem:int) (arr:list) =     
     let filteredList = arr |> List.filter (fun x -> x = num)
     if filteredList.Length > 1 || num = 0 then
          arr.Length - 1
     elif (num*10) < rem then
          remainders (num*10) rem arr
     else
          let newRem = (num*10) % rem
          let newArr = List.append arr [newRem]
          remainders newRem rem newArr


let tempstart = System.DateTime.UtcNow


let remaindersParallel =
     [|2..999|] |> Array.Parallel.map (fun x -> remainders 1 x [])


let remaindersMax arr = Array.max arr


let remaindersIndex arr max =
     let idx = arr |> Array.findIndex (fun x -> x = max)
     idx+1


let result =
     let remainders = remaindersParallel
     let max = remaindersMax remainders
     let idx = remaindersIndex remainders max
     (idx, max)